Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have utility as adsorbent materials and to have catalytic properties for various types of hydrocarbon conversion reactions. Certain zeolitic materials are ordered, porous crystalline metallosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as “molecular sieves” and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO4 tetrahedra and optionally tetrahedra of a Periodic Table Group IIIA element oxide, e.g., AlO4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIA element is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983); ZSM-35 (U.S. Pat. No. 4,016,245); zeolite ZSM-23 (U.S. Pat. No. 4,076,842); zeolite MCM-22 (U.S. Pat. No. 4,954,325); and zeolite MCM-35 (U.S. Pat. No. 4,981,663), merely to name a few.
Type “Y” zeolites are of the faujasite (“FAU”) framework type which is described in Atlas of Zeolitic Framework Types (Ch. Baeriocher, W. M. Meier, and D. H. Olson editors, 5th Rev, Ed., Elsevier Science B.V., 2001) and in the pure crystalline form are comprised of three-dimensional channels of 12-membered rings. The crystalline zeolite Y is described in U.S. Pat. No. 3,130,007. Zeolite Y and improved Y-type zeolites such as Ultra Stable Y (“USY” or “US-Y”) (U.S. Pat. No. 3,375,065) not only provide a desired framework for shape selective reactions but also exhibit exceptional stability in the presence of steam at elevated temperatures which has resulted in this zeolite structure being utilized in many catalytic petroleum refining and petrochemical processes. Additionally, the three-dimensional pore channel structure of the faujasite framework zeolites, such as the Y-type zeolites, in combination with their relatively good ability to retain a high surface area under severe hydrothermal conditions and their generally low cost to manufacture makes these zeolites a preferred component for Fluid Catalytic Cracking (“FCC”) catalysts in petroleum refining and petrochemical processes.
In a pure zeolite crystal, the pore diameters are typically in the range of a few angstroms in diameter. Y-type zeolites exhibit pore diameters of about 7.4 Angstroms (Å) in the pure crystal form. However, in manufacture, defects in the crystalline structure and in particular in the inter-crystal interfaces occur in the crystalline structure of zeolites, including the Y-type zeolites. Additionally, due to certain methods of preparations and/or use, both wanted and unwanted structural modifications can be made to the zeolite crystal. It is these “defects” which lead to specific properties of the zeolite which may have beneficial properties when utilized in catalytic processes.
The conventional Ultra Stable Y (USY) zeolites prepared by mild steam calcination, as taught by U.S. Pat. No. 3,375,065, contains mesopores in the 30 to 50 Å regions. Pores with pore diameters in the 30 to 50 Å range are herein defined as “Small Mesopores”. Another type of Y zeolite stabilization utilizes chemical processes to remove framework aluminum atoms. One example of Y zeolites obtained from such processes is LZ-210 (U.S. Pat. No. 4,711,770). In LZ-210, the vacancies of removed aluminum atoms are replaced by silicon atoms, therefore preserving nearly perfect crystal structure of Y zeolite with very little formation of mesopores. In FCC applications, however, such perfect Y zeolite, i.e., without mesopores, leads to low conversions of heavy hydrocarbons. As the FCC feed stream is getting heavier, it is more desirable to have a zeolite with more mesopores in the large mesoporous region. Here we define “Large Mesopores” as pores with pore diameter in the greater than 50 to 500 Å regions. It is believed that zeolites with large mesopores can enhance conversions of heavy hydrocarbons. A problem that exists in the industry is that many Y-type zeolites (e.g., Na—Y zeolites), while widely used in the industry, exhibit a “peak” in the small mesopore range (30 to 50 Å pore diameters) while exhibiting no significant pore volume associated with the large mesopore range (50 to 500 Å pore diameters). Conversely, other Y-type zeolites (e.g., USY zeolites), exhibit a significant “peak” in the small mesopore range (30 to 50 Å pore diameters) when some large mesopores are present.
Therefore, what is needed in the art is an improved Y-type zeolite which possesses an improved large mesoporous volume to small mesoporous volume ratio structure while suppressing the magnitude of the “small mesopore peak” associated with pores measured in the small mesopore range (30 to 50 Å pore diameters)